An inconsistency between explicit and implicit hashing when signing in Java security?

For the connection to a certain other system within our network, the program I'm working on needs to verify that it indeed is what it claims to be: an authorized client. A common way to accomplish is through PKI: it signs the message it sends using a private key, and the other system can verify this signature using the corresponding public key. See e.g. this article for an explanation of how this works.

In our case, there are three steps in signing a message:

• calculation of the message digest through a hashing algorithm,
• calculation of the digital signature using the private key, and
• coding the result to base64.

The last step is not part of the normal signing process, but we need to send the result as a string inside an XML message. Using the 'raw' signature would result in weird characters in the XML, very likely choking up the parser.

As I was coding away, I was lulled into performing each of these steps separately, so I started off with implementing the hashing using the java.security.MessageDigest class. I instantiated it with the "SHA-1" algorithm and simply called the digest method with the message to obtain
its hash. Pretty straightforward stuff.

Then I turned to the java.security.Signature class to supply me with the subsequent signing functionality. It occurred to me that there are algorithm choices that include hashing algorithm names, so I quickly found out that it is possible to let the Signature class take care of
both the first and second step of my signing process. While that struck me as quite convenient, I decided to stick to the original plan and not use the hashing possibility here. I chose the "NONEwithRSA" algorithm, and after feeding the message and private key the sign method provided me with an answer.

Then I encoded it in base64 (using the Apache Commons Codec library, which I also could have used for the hashing functionality) and presto! So I thought, at least...

But then...

The first test we performed immediately indicated something was wrong. And after checking everything else (like making sure code page encodings were correct and what not) we came to the conclusion that the signature itself had to be the culprit.

So I decide to put the 'my' way of creating a signature side by side with the signing method using the implicit hashing possibility, to see whether there might be a difference in the outcome:

public void test(byte[] data, PrivateKey privateKey) throws Exception {
   // Explicit hash and separate signing:
   byte[] hashedData = MessageDigest.getInstance("SHA-1").digest(data);
   byte[] signedData = signData(hashedData, privateKey, "NONEwithRSA");

   // Signing with implicit hashing:
   byte[] signedHashedData = signData(data, privateKey, "SHA1withRSA");

   System.out.println("Encoded data (explicit hashing) = "
           + new String(Base64.encodeBase64(signedData)));
   System.out.println("Encoded data (implicit hashing) = "
           + new String(Base64.encodeBase64(signedHashedData)));
}

private static byte[] signData(byte[] data, PrivateKey privateKey, String algorithm) throws Exception {
   Signature signature = Signature.getInstance(algorithm);
   signature.initSign(privateKey);
   signature.update(data);
   return signature.sign();
}

And even though:

• from the code it seems that both paths should lead to the same result: no other configuration than the algorithm names are given, so all else should be default, and
  
• from the Java security documentation for the signing options "NONEwithRSA" is stated to 'not use a digesting algorithm', so it should act as "SHA1withRSA" without the SHA-1 hashing,
  

there definitely is a difference between the outcomes!

In our situation, the implicit hashing turned out to deliver the correct result (at least, with regards to what the other system expected), so a minimal code change (getting rid of the explicit hashing step) did the trick. We use an external configuration file to set the signing algorithm through a system property, so changing that was easy.

What's causing this?

Now why is there a difference between the two approaches? I've tried to find an answer using Google, but that quest didn't turn up any answers.
So I did what any self-respecting developer would do: step through the implementation in a debugger. Unfortunately my toolkit didn't allow me to see everything I wanted to; however I could see that the input of the signing step was identical in both cases and the same implementation for signing is used under the hood (sun.security.rsa.RSACore). I cannot see what is happening with respect to internediate padding of the byte arrays, however, so I'm guessing that the 'default' settings of the two approaches - driven by different SignatureSpi implementations - differ in this respect.

If anyone could point out the actual difference to me, that would be greatly appreciated. For now I'll have to be content with knowing that the two approaches do return different results and that picking one at random may lead to problems...

Blind spot in the Java API: dynamic proxies

Reflecting on the good ol' days

Back in the days that the reflection API was added to Java, it meant there was a new way to do much of the same, yet in a very generic and nifty way. That came of course at a price: it slowed things down considerably - which was significant as the performance of Java as a whole (language + platform) was still quite an issue at the time.

In those days I was still working in the Telecom industries, where Java just began to get some foothold. Most of the applications running on the network nodes were still written in C++ (or even plain C) and were simply faster. The powers-that-be decided against the use of reflection (at least in production code) for exactly those performance related reasons and therefore my attention - after a short first impression - shifted away from that API.

Back to the present

Last week at work I came across some code a co-worker had written and which I was about to use for 'inspiration'. One class implemented some interface named InvocationHandler which I'd never seen before and (obviously) used reflection. I looked it up in the Sun Javadocs and found to my surprise that this interface (along with its partner in crime, the class Proxy) have been around since Java 1.3! That means over eight years - Ouch!

Ever since I read Scott Meyers books on C++ (yes, this one in particular) I know there is great advantage in being aware of the available libraries. And even though Scott's remark only seemed to apply to the standard library for C++, I'm sure he meant this in a much wider context. Any engineer should know his tools.

Knowing all of Java's APIs is another thing altogether, though. The list of classes and interfaces available in the Standard Edition alone seems to be endless, and adding the Enterprise Edition and Micro Edition to the mix makes for a sheer impossible task. I can't imagine there's a soul alive who actually does know about all of them - and knows how to use them in a practical manner, of course.

It has never been my goal to learn all about the Java APIs, just the parts that I need to write efficient code. But this is one I definetely missed...

So what's it all about then?

First off, it's called 'proxy' because it allows you to implement the Proxy design pattern (actually it's not hard to pull off a Decorator implementation either), and it's called 'dynamic' because it implements a given interface at runtime - and that's where reflection comes into play.

This is the 'classic' Proxy pattern:
A dynamic proxy doesn't have the compile time relationship with the interface. As said, this relationship is forged at runtime. The code for such a class might look something like this:

package server;

import java.lang.reflect.*;

public class DynamicProxy implements InvocationHandler {
    private Object realSubject;

    public DynamicProxy(Object realSubject) {
        this.realSubject = realSubject;
    }

    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable {
        System.out.println("Before the call...");
        Object result = method.invoke(realSubject, args);
        System.out.println("...after the call.");
        return result;
    }
}

Of course you may want to insert something more useful before and after the method invocation in a practical case...

Now all one needs is the means to let the client get a reference to a Subject in a way that doesn't reveal the dynamic proxy. A factory, which normally may just hide the implementation class and/or static proxy from the client, could be 'upgraded' to do just that:

package server;

import java.lang.reflect.Proxy;

public class SubjectFactory {
    public static Subject getSubjectInstance() {
        return (Subject) Proxy.newProxyInstance(RealSubject.class
            .getClassLoader(), new Class[] { Subject.class },
            new DynamicProxy(new RealSubject()));
    }
}

Notice how:

• the classloader of the implementation class is given as the first argument
• the Subject interface is entered as the second parameter; in fact all (distinct) interfaces implemented by the implementation class may be given here, so RealSubject.class.getInterfaces() would be fine too
• the DynamicProxy is instantiated given a RealSubject instance; this is the instance on which the actual invocation is performed

And this is what the class diagram has become:

The definitions of the Client and ServerImpl classes and Server interface haven't changed at all if a factory was used in the first place. So this little mechanism can be used to insert any pre- and post-processing you need at any time, e.g. to surround the call with session or transaction management.

Now do I hear someone say AOP? This construct actually predates AspectJ, and I guess that if you don't need all of the versatility of such a framework, it's good to know this tool is in your standard toolkit!

If this short intro has grabbed your interest, you may want to go on reading this old Javaworld article about dynamic proxies to get your feet wet a little more...